1. Field of the Invention
This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for sensing current in a subscriber line card.
2. Description of the Related Art
In communications systems, particularly telephony, it is a common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office through a subscriber line. At the subscriber end, a telephonic device may be employed to establish communication with a remote user using the subscriber line. The combination of the telephonic device and the subscriber line is commonly referred to as a subscriber loop.
A line card generally includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The primary functions of the line card may include supplying talk battery, performing impedance matching, determining whether telephonic equipment is on-hook or off-hook, and handling ringing signals, voice signals, and testing signals.
Typically, a line card senses voice currents on the subscriber line by amplifying the voltage drops across resistors (commonly referred to as xe2x80x9csense resistorsxe2x80x9d) in series with the line driver. This technique, however, has at least one shortcoming in that the sense resistors create an excessive voltage drop in the subscriber loop, especially if large sense resistors are employed. Furthermore, handling these large voltage drops across sensing resistors requires use of voltage amplifiers, which require larger current draw from a battery supply or other such circuitry. One way to avoid the large voltage drops is to utilize smaller sense resistors. However, when smaller sense resistors are employed, the available signal is also small, thereby making the available signal more susceptible to noise.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
In one aspect of the present invention, a method is provided for sensing current in a subscriber line. The method includes receiving a current signal from a driver capable of driving the current signal on the subscriber line, and generating a second signal in response to receiving the current signal, wherein the second signal is proportional to the current signal. The method includes receiving at least a portion of the current signal from a driver capable of sinking the portion of the current signal from the subscriber line, and generating a third signal in response to receiving at least the portion of the current signal, wherein the third signal is proportional to the portion of the current signal. The method further includes adding the second signal and the third signal to generate an output signal.
In another aspect of the present invention, a circuit is provided for sensing current in a subscriber line. The apparatus includes an output driver adapted to drive a current signal on the subscriber line, and first circuitry coupled to the output driver and adapted to generate a second signal in response to receiving the current signal, wherein the second signal is proportional to the current signal. The circuit includes second circuitry adapted for receiving at least a portion of the current signal, and third circuitry coupled to the second circuitry and adapted to generate a third signal in response to receiving at least the portion of the current signal, wherein the third signal is proportional to the portion of the current signal. The circuit further includes fourth circuitry capable of adding the second signal and the third signal to generate an output signal.